Along with a rapid spread of information relevant apparatuses and communication apparatuses such as a personal computer, a video camera and a portable telephone in recent years, the development of a battery to be utilized as a power source thereof has been emphasized. Also in the automobile industry, the development of a high-output and high-capacity battery for an electric automobile or a hybrid automobile has been advanced. A lithium secondary battery has been presently noticed from the viewpoint of a high energy density among various kinds of batteries.
Since liquid electrolyte containing a flammable organic solvent is used for a presently commercialized lithium secondary battery, it is necessary to install a safety device for restraining temperature rise during a short circuit and to improve the structure and the material for preventing the short circuit. On the contrary, a lithium solid secondary battery all-solidified by replacing the liquid electrolyte with a solid electrolyte layer is considered to have an advantage of the simplification of the safety device and to be excellent in production cost and productivity because it does not need any flammable organic solvent therein.
A secondary battery can be repeatedly charged and discharged, but it is also known that the battery performance thereof could deteriorate through over-discharge process. Thus, a normal secondary battery has a structure for measuring the voltage of the battery during the discharge in order to stop the discharge at a predetermined voltage. In the meantime, Patent Reference-1 discloses a battery module which does not have any over-discharge protecting means for preventing the over-discharge of the lithium secondary battery, and Patent Reference-2 discloses an electrical apparatus which does not have any over-discharge protecting means for preventing the over-discharge of the lithium secondary battery.